The present invention relates generally to plant cutter apparatus for cutting unwanted plants such as bushes, weeds or grass, and more particularly to an improved drive shaft for use in plant cutter apparatus to transmit driving force of a prime mover or drive source unit, such as an engine or electric motor, to a cutter blade.
Generally, weeds growing on footpaths between rice fields etc. have to be cut several times a year because they tend to be nested by pests. The weed cutting is usually very laborious, and thus various automatic weed cutters have heretofore been proposed and put to practical use, among which shoulder-hung weed cutters are very popular because of their small size and handling ease. In most of the shoulder-hung weed cutters, a drive shaft or driving-force transmission shaft, which is passed through a handling rod, is rotated via an engine provided at one end of the handling rod so as to rotate a cutter blade provided at the other end of the handling rod. In most cases, a human operator hangs the weed cutter on his or her shoulder using a hanging belt, and cuts weeds with the rotating cutter blade by gripping a U-shaped handle provided on an intermediate position of the handling rod to swing the handling rod in front-and-rear and left-and-right directions.
In recent years, there have been strong demands for further improvements in agricultural working environment. In the case of the shoulder-hung weed cutters, there is an increasing demand for reduction in vibrations that are transmitted from the driving-force transmission shaft via the handling rod to the U-shaped handle, with a view to lowering the load on the human operator. The driving-force transmission shaft, passed through the handling rod, has a relatively great length and is thus liable to sag relatively greatly due to its own weight, which would result in so-called xe2x80x9csagging-induced vibrationsxe2x80x9d or xe2x80x9csagging vibrationsxe2x80x9d when the engine is activated. It has been conventional to minimize the sagging vibrations using any one of the following two measures. The first measures are to support the driving-force transmission shaft via a number of bearings disposed on the shaft and spaced apart from each other along the shaft""s axis in such a manner that small shaft lengths are supported separately between the bearings, and the second measures are to reduce the weight of the driving-force transmission shaft. The first measures, however, are not satisfactory in that the provision of the many bearings presents structural limitations and considerably increases the overall weight of the weed cutter. Therefore, the second measures of reducing the weight of the driving-force transmission shaft itself are being given more consideration today. Specifically, in order to reduce the weight of the conventional driving-force transmission shaft that is in the form of a steel rod or steel pipe, it is conceivable to form the driving-force transmission shaft into a smaller diameter or into a pipe shape having a small wall thickness or make the transmission shaft of a light weight material. However, because the function of the driving-force transmission shaft is to transmit the driving source from the engine to the cutter blade, the driving-force transmission shaft has to have more than predetermined rigidity against torsional and bending force, and thus the option of forming the driving-force transmission shaft into a smaller diameter or into a pipe shape having a small wall thickness is subject to limitations. Further, as the option of making the transmission shaft of a lightweight material, it is conceivable to use an aluminum alloy material; however, such an aluminum alloy material can not achieve necessary rigidity although it is, to be sure, light in weight.
Specific examples of the weed cutters where the driving-force transmission shaft is made of a lightweight material are known, for example, from Japanese Utility Model Publication No. HEI-1-30995 and Japanese Patent Laid-Open Publication No. HEI-8-205649.
Namely, in the weed cutter disclosed in the HEI-1-30995 utility model publication, the driving-force transmission shaft, passed through the handling rod formed into a pipe shape, is driven via the drive source unit to rotate the cutter blade mounted at the distal end of the handling rod. Here, the transmission shaft is made of a resin material lighter in weight than the conventional steel material, in order to reduce the undesired sagging vibrations of the shaft. However, because the resin-made transmission shaft generally has low torsional and bending rigidity, there is a need to make up for the lack of the rigidity in some way. If the diameter of the transmission shaft is increased to secure the necessary rigidity, the weight saving objective can not be accomplished. Further, according to the disclosure of the HEI-1-30995 utility model publication, it is not clear how the transmission shaft is connected to the output shaft of the drive source unit and cutter blade. Furthermore, since the output shaft of the drive source unit and cutter blade are metal-made component parts, it is not easy to reliably connect the resin-made transmission shaft to the metal-made output shaft and cutter blade with sufficient coupling strength.
In the plant cutter disclosed in the above-mentioned HEI-8-205649 patent laid-open publication, the driving-force transmission shaft is in the form of a pipe made of fiber-reinforced resin. Metal-made joints are fitted in and adhered to the opposite ends of the pipe-shaped transmission shaft by an adhesive agent. Outer peripheries at the ends of the transmission shaft, to which the metal-made joints are adhered, are tightened by means of reinforcing rings so as to more firmly fix the joints and thereby increase the over all rigidity of the pipe-shaped transmission shaft made of the fiber-reinforced resin. However, if the same rigidity as the steel is required of the thus-constructed transmission shaft (i.e. pipe made of the fiber-reinforced resin), there would unavoidably arise a need to increase the diameter and wall thickness of the transmission shaft. Increasing the diameter of the transmission shaft is unadvisable because it also requires an increase in the diameter of the handling rod through which the transmission shaft is passed. Further, fixing the metal-made joints to the pipe-shaped transmission shaft made of the fiber-reinforced resin in the above-mentioned manner would result in a complicated structure. In addition, the transmission shaft can not achieve sufficient durability at and around the ends where the metal-made joints are fixed to the resin-made transmission shaft.
In view of the foregoing, it is a first object of the present invention to provide a plant cutter apparatus which can minimize vibrations of a drive shaft while maintaining necessary drive shaft""s rigidity without having to increase the diameter of the drive shaft.
It is a second object of the present invention to provide a plant cutter apparatus which allows a drive shaft to be reliably connected to the output shaft of a drive source unit and a cutter blade and which can achieve sufficient durability in portions of the transmission shaft that are connected to the output shaft of the drive source unit and cutter blade.
According to the present invention, there is provided an improved plant cutter apparatus which comprises a pipe-shaped handling rod, a drive shaft passed through the pipe-shaped handling rod, a drive source unit mounted at one end of the handling rod for rotating the drive shaft and a cutter blade mounted at the other end of the handling rod for being rotated by the rotation, via the drive source unit, of the drive shaft, and which is characterized in that the drive shaft is a different-metal-combined shaft that includes a main shaft portion made of a titanium alloy, and end shaft portions made of steel and integrally joined to the opposite ends of the main shaft portion.
The titanium alloy has substantially the same mechanical strengths (torsional strength, bending strength, longitudinal elasticity coefficient, etc.) as the steel, and has a specific gravity smaller than that of the steel. Particularly, because the main shaft portion is pipe-shaped, it can be even lighter in weight. Therefore, the titanium-alloy-made main shaft portion can sufficiently secure torsional and bending rigidity that is normally required of the plant cutter""s drive shaft, even where the main shaft portion has the same diameter as the conventional counterparts. Further, the use of the lightweight titanium-alloy-made main shaft portion can significantly reduce unwanted xe2x80x9csagging-induced vibrationsxe2x80x9d of the drive shaft that tend to be caused by the shaft""s own weight.
Furthermore, with the inventive arrangement that the steel-made end shaft portions are integrally joined to the opposite ends of the titanium-alloy-made main shaft portion, it is possible to secure durability in the portions of the drive shaft where the end shaft portions are joined to the main shaft portion. In addition, the opposite ends of the drive shaft, i.e. the two end shaft portions, can be coupled reliably to the output shaft of the drive source unit and the cutter blade with sufficient durability. It is preferable that the two end shaft portions be made of carbon steel for machine structural use.
Preferably, the end shaft portions are fitted in the opposite ends, respectively, of the main shaft portion and integrally joined to the respective ends by vacuum brazing. Thus, the drive shaft can secure sufficient rigidity against any possible bending force applied to the portions where the end shaft portions are fitted in the main shaft portion.